1. Field of the Invention
The present invention relates to prevention of false, annoying, or oversensitive alarms during medical procedures, providing early detection by a sensitive test, generating silent, semi-overt, or overt alarm conditions and/or initiating early passive or active interventions to untoward events.
2. Description of Related Art
In certain clinical incidents or emergencies, timely intervention may be critical to outcome. Earlier detection of a developing untoward clinical event facilitates timelier diagnosis and intervention and enhances the probability of a safe and minimally disruptive recovery. Sensitive tests and alarms, in general, assist in earlier detection. However, sensitive tests and alarms are also more prone to annoying, distracting and potentially disruptive false positive alarms. Thus, a medical device designer (or clinician in the case of user-adjustable alarms) generally compromises in setting alarm thresholds so that false positive alarms are minimized and true alarm conditions are detected. Much valuable time may be lost due to this compromise.
Correct assessment of gas exchange during procedures involving sedation and analgesia is important because respiratory depressants are often administered to patients undergoing painful medical procedures. Respiratory depressants, such as sedation and analgesia agents, can relax the soft tissue of the throat causing partial or complete airway obstruction in some patients, or blunt the respiratory drive, i.e., the urge to breathe when the blood level of carbon dioxide rises. If not diagnosed promptly, such conditions can quickly develop into a life-threatening situation. If a patient does not move a sufficient volume of gas containing oxygen into and out of the lungs then the patient will develop a deficiency in the oxygen supply to body tissue (hypoxia) which, if severe and progressive, is a lethal condition.
In many health care settings, clinicians assess respiratory gas exchange by using an elevated arterial partial pressure of carbon dioxide (PaCO2) as an indicator of incipient respiratory failure or prolonged airway obstruction. In this regard, the determination of PaCO2 is useful in optimizing the settings on ventilators, detecting life-threatening blood gas changes, and detecting the presence of airway obstruction in an anesthetized or sedated patient undergoing a medical procedure. The traditional method of obtaining arterial blood gas values is to extract a sample of arterial blood and measure PaCO2 using a blood gas analyzer. Arterial puncture with a needle to extract the arterial blood sample has inherent limitations: 1) arterial puncture carries a degree of patient discomfort and risk, 2) handling of the blood is a potential health hazard to health care providers, 3) significant delays are often encountered before results are obtained and, 4) measurements can only be made intermittently. Furthermore, blood CO2 measurements do not immediately reflect changes in patient ventilation, so they may not detect airway obstruction in its early stages when it may still be corrected prior to the onset of adverse physiological consequences. Therefore, clinically, early or timely detection of hypoventilation via blood gas analysis is not practical and this approach might even be considered unsafe and ineffective.
Hypoventilation results from low or no minute ventilation (MV). Minute ventilation is the product of respiratory rate (RR) and tidal volume (VT). Low MV may be caused by bradypnea (low RR) or apnea (no breathing; RR=0) or inadequate tidal volumes (resulting from, among others, airway obstruction, shallow breathing, insufficient VT, VT less than dead space) or a combination of low VT and low RR. A fast RR does not exclude hypoventilation if VT is too small for effective ventilation of the lungs or less than the deadspace. Similarly, a large VT does not exclude hypoventilation if RR is too low for adequate minute ventilation.
Continuous invasive monitoring requires in-dwelling arterial lines that entail inherent problems such as, for example, sepsis or thrombosis. The nature and expense of this monitoring system excludes its application under routine care, restricting its use to intensive care units within a hospital facility. In-dwelling arterial lines providing real-time PaCO2 analysis are not able to tell the immediate status of a patient's ventilation, because there is a time delay between the onset of ventilatory insufficiency or hypoventilation and a subsequent rise in arterial carbon dioxide levels.
In current clinical practice, PaCO2 levels are indirectly inferred via capnometry, the measurement of CO2 levels in the gas mixture breathed by a patient. If the CO2 levels, in addition to being measured, are also graphically displayed as a CO2 level vs. time plot, the technique is called capnography and the resulting plot is called a capnogram. A typical capnogram comprises three distinct phases during exhalation. Phase I reflects the clearing of CO2-free gas from conducting airways which do not normally participate in gas exchange (i.e., airway dead space). Phase II is generated by exhalation of CO2-free gas from conducting airways mixed with alveolar gas containing CO2 because the alveolar gas has undergone gas exchange with arterial blood containing CO2 at the alveolar membrane. Phase III reflects the exhalation of alveolar gas which has had time, through the process of diffusion, to equilibrate its partial pressure of CO2 with the partial pressure of CO2 in arterial blood.
Because the lung's airways are a dead-ended conduit, gas flow in the lungs follows a first in, last out principle. Thus the last amount of alveolar gas exiting the lungs during exhalation was the first in and has had the most time to equilibrate its partial pressure with the partial pressure of the equivalent substance in arterial blood, such as, among others, CO2, O2, volatile anesthetic, intravenous anesthetic, alcohol, medication and inert gas anesthetic. Thus, in healthy patients, alveolar gas exhaled during phase III is representative of the partial pressures of different substances dissolved in arterial blood. Further, the CO2 component of alveolar gas exhaled during Phase III is generally a good indicator of the ventilatory status of a healthy patient.
When using capnometry or capnography, clinicians generally utilize the peak or end-tidal CO2 (PetCO2) value as an estimate Of PaCO2. PetCO2 is indicative of the mean alveolar partial pressure of carbon dioxide from all functional gas exchange units of the lung, which, in turn, approximates PaCO2 in normal lungs. Because CO2 readily diffuses from arterial blood into alveolar gas across the alveolar membrane, PetCO2 closely approximates PaCO2 when the lung has normal ventilation and perfusion. In addition to the information provided by the PetCO2, the shape of the capnogram also provides valuable diagnostic information regarding the respiratory ventilation.
Other techniques have been utilized for assessing patient blood gas levels with mixed results. Transcutaneous CO2 sensors measure the partial pressure of CO2 in tissue. The sensors are placed onto the skin of the patient and measure CO2 diffusing through heated skin but have practical and theoretical limitations. Pulse oximetry is a widely used, non-invasive method for estimating the arterial oxygen carried in hemoglobin. Neither transcutaneous measurements of CO2 nor pulse oximetry directly measures and reports the status of respiratory ventilation. Thus, transcutaneous CO2 measurement and pulse oximetry may be late to diagnose an impending problem. In the case of pulse oximetry, once the condition of low oxygen is detected, the problem already exists, and once the transcutaneous CO2 measurement is elevated, it indicates that hypoventilation has already existed for a period of time sufficient for a rise in the partial pressure of tissue CO2.
Capnometers have been used with some success as a means for detecting and avoiding the severe complications associated with hypoventilation, partial or complete airway obstruction, bradypnea and apnea. Systems assessing proper gas exchange based on predetermined or user-adjusted carbon dioxide thresholds detect instances of hypoventilation or airway obstruction. In general, the CO2 level must exceed a lower threshold (indicating sufficient gas exchange and ruling out apnea) and stay below a higher threshold (indicating adequate ventilation and ruling out high end-tidal CO2 concentrations due to, for example, hypoventilation). However, capnometers are often prone to false positive alarms.
A false positive alarm occurs when a system indicates that a potentially dangerous situation has arisen, when in fact, it has not. False positive alarms may occur in situations where a change in CO2 levels is unrelated to respiratory gas exchange. Such misleading alarms may result from a patient talking, breathing through an unmonitored orifice, or dilution of the exhaled gases at the sampling source. False positive alarms may occur in systems where a predetermined carbon dioxide threshold may be set at an arbitrary point that may not be representative of inadequate gas exchange. Systems prone to false positive alarms are often deactivated by clinicians or simply ignored, putting a patient at risk if a truly life threatening situation occurs.
During inhalation, a patient breathing ambient air will inhale room air containing a negligible amount of carbon dioxide (0.03% v/v) that will not register on clinical capnometers. The beginning of an exhalation may be nearly indistinguishable from the inhalation phase due to a patient breathing out dead space gas that has not mixed with alveolar CO2 found deeper in the lungs. As a patient continues to exhale, alveolar CO2 will be expelled from the lungs and the CO2 level will cross a lower threshold as he/she continues to exhale, eventually reaching a plateau or peak referred to as “end-tidal” CO2. As a patient begins to inhale, carbon dioxide levels will drop below the lower threshold level due to a negligible amount of CO2 in room air. The period between a crossing of a threshold on an exhalation upstroke and a crossing of the same threshold on a subsequent exhalation upstroke is usually considered as a full breath or respiratory cycle.
When hypoventilation is due to adequate VT but low RR, the CO2 level will cross a lower CO2 threshold and eventually the PetCO2 will exceed a higher CO2 threshold as alveolar CO2 concentration rises because CO2 is accumulating in the alveoli as a result of inadequate minute ventilation. When hypoventilation is due to adequate RR or fast RR but low VT (shallow breathing or panting), the CO2 level may never cross the lower CO2 threshold because the exhaled gas is comprised mainly of dead space gas devoid of CO2 and is at most mixed with a minimal amount of alveolar CO2.
Many CO2 monitoring systems are programmed to initiate an alarm in the event that a patient does not complete a sufficient number of respiratory cycles (breaths) within a predetermined time window. False negative alarm conditions may result from such systems, where inadequate gas exchange is occurring in a patient but a system fails to recognize a potentially life threatening event. The fact that the exhaled CO2 level crosses a lower CO2 threshold within a predetermined time window is not sufficient to assure that a patient is experiencing adequate gas exchange. For example, a patient with a significant partial airway obstruction may break through a blockage in order to take a short (physiologically insignificant) breath, registering with a capnometer system that a patient is breathing at a normal rate within a predetermined time window such that an airway obstruction may remain undetected. Breaths taken by a patient, though of normal frequency, may not be of adequate volume to provide sufficient oxygen supply and carbon dioxide elimination to maintain a healthy state.
An untoward event will usually generate an alarm to alert a clinician. Generally, a clinician will respond to an alarm by taking an appropriate corrective action. Thus, an untoward event generates two distinct actions: an alarm (usually automated) and a response (usually manual but it may also be automated). The terms “response” and “alarm” will be used consistently herein according to the definitions above. The response of a clinician usually also involves turning off the alarm because of its annoying nature, requiring a superfluous action that does not directly contribute to patient care. In the event of a false positive alarm, even more time and motion are wasted in activities that do not directly contribute to, and may detract from, patient care. False alarms may also devalue the benefit and credibility of alarms (the “cry wolf” syndrome).
With some systems featuring automated responses (such as interruption of drug delivery) to alarms, an audible or visual alarm generally accompanies an automated response. A design rationale for having an overt alarm (a potential annoyance) generally accompany an automated response (a potential benefit to a busy, multi-tasked clinician) is that an untoward condition should not be masked from a clinician, even if the system has initiated an automated corrective response. Therefore, a tightly set automated response that is designed to intervene early and/or frequently to provide better control of a given parameter will, in general, also generate more frequent and potentially disruptive alarms.
In the past, increasing the sensitivity of monitoring systems created a greater probability of detecting untoward events but also increased the probability of false alarms triggered by patient conditions that do not warrant the attention of a busy, multi-tasked clinician. Decreasing the sensitivity of monitoring systems diminishes the incidence of false alarms but increases the probability that critical untoward events may be missed.
False positive alarms may be caused by an over-sensitive alarm algorithm that is vulnerable to spurious data or a data artifact. Over-sensitivity may be due to a short averaging period, or no averaging, of carbon dioxide. False negative alarms are generally attributable to low specificity, where specificity relates to determining the actual significance of information received via patient monitoring. High specificity may reduce alarms associated with spurious data or over-sensitivity, yet may also hide those patient episodes that constitute truly life-threatening situations.
A further example of potential false negative alarm episodes occurs when a patient experiences ineffective hyperventilation, characterized by high respiratory rates with very low tidal volumes. Breathing at very low tidal volumes expels mainly dead space gas from the upper airway that has not, or minimally, mixed with alveolar CO2. The next inhalation of a small tidal volume is sequestered in the dead space formed by the upper airway and never or barely reaches the alveoli where gas exchange occurs. During hyperventilation, a carbon dioxide threshold may be just reached, indicating a breath to the CO2 monitor. However, a patient may not be inhaling sufficient oxygen or eliminating sufficient carbon dioxide for adequate gas exchange.
Because even short acting drugs exhibit a finite half-life, it is desirable to reduce or shut off drug delivery as early as possible in the event of an untoward patient state, providing in effect an “early response” system so that an untoward condition can be promptly reversed. In the context of systems integrating ventilatory monitoring and sedative and/or analgesic drug delivery, deactivation of drug delivery early in the development of a life-threatening condition is desirable.
It would therefore be advantageous to provide a respiratory gas exchange monitoring system for detecting partial or complete airway obstruction or depressed respiratory drive to breathe comprising high sensitivity and high specificity, thus diminishing the incidence of false positive and false negative alarms. It would be even further advantageous to provide a respiratory gas exchange monitoring system integrated with a sedative and/or analgesic drug delivery system that deactivates drug delivery at the onset of a potentially dangerous patient episode.
It would be further advantageous to provide a respiratory gas exchange monitoring system that accurately measures and indicates carbon dioxide elimination during every breath. It would be further advantageous to provide a respiratory gas exchange monitoring system that is capable of estimating overall carbon dioxide elimination during a procedure in such a way as to determine whether exhaled carbon dioxide levels are relatively constant. It would be further advantageous to provide a respiratory gas exchange monitoring system integrated with a drug delivery system designed for operation by non-anesthetists that provides additional patient safety features.